Variable Refrigerant Flow Cooling System

ABSTRACT

A variable flow refrigerant system having a compressor and one or a plurality of evaporators. The suction at one or the plurality of evaporators for the input to the compressor is monitored and generally corresponds to the minimum pressure of the refrigerant. The pressure is associated with a temperature and is controlled to always be above the dew point temperature of the room.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/472,723 filed on Apr. 7, 2011. The entire disclosure of the aboveapplication is incorporated herein by reference.

FIELD

The present disclosure relates to a variable refrigerant flow coolingsystem.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

One way to broadly characterize technology for cooling room spaces iscomfort cooling, typically for residential or office spaces occupied bypersons, and industrial or electronics cooling, typically applicable toroom spaces containing systems that generate significant amounts ofheat. For example, electronic data centers can include server rooms,telecommunication rooms, or other spaces which house multipleelectronics systems. Such systems are densely arranged with electronicequipment that generates significant amounts of heat. Cooling thesespaces requires a substantially greater capacity than is typicallyrequired in a conventional residential or office space.

One configuration for comfort cooling applications, such as forresidential or office spaces, utilizes variable refrigerant flow (VRF)technology. A conventional VRF system includes an outdoor unit andmultiple indoor units. The outdoor unit can include compressors andcondensers, while the indoor unit typically includes an expansiondevice, a heat exchanger, such as a microchannel heat exchanger, and afan. The compressors are typically embodied as variable capacitycompressors. Various techniques are used to control the overall capacityof the compressor based on the sum of the loads of the indoor units. Therefrigerant flow is varied in order to minimize the load on thecompressor and increase efficiency.

Cooling units configured to provide cooling to room spaces having highheat producing equipment are typically configured differently. Invarious configurations, the cooling systems may include two distinctcircuits, each circuit utilizing different refrigerants and mechanicalparts. A first circuit may be a pumped circuit that contains redundantcirculating pumps, and in various configurations, a brazed plate heatexchanger along with accompanying valves and piping. A second circuitmay be configured in a dual direct expansion circuit containing scrollcompressors, expansion valves, brazed plate heat exchangers, and variouspiping. The brazed plate heat exchanger provides the interface betweenthe two circuits. Heat rejection is accomplished by using condensersconnected to the dual direct expansion circuit. In such conventionalcooling units, dew point control is achieved by monitoring refrigeranttemperature in the pumped circuit and backing off the compressors in thedual direct expansion circuit if the temperature gets too low. Thiseffectively reduces the capacity of the dual direct expansion circuit inorder to maintain a minimum refrigerant temperature in the pumpedcircuit. In other systems, the temperature of the pumped circuitrefrigerant is controlled using the flow rate of the fluid that removesheat from the circuit. For example, various systems modulate a chilledwater valve in order to maintain dew point margin of a refrigerant wherethe refrigerant is cooled by the building chilled water system.

In various configurations, a cooling system designed for high heatproducing environments monitors room conditions and preventscondensation by maintaining the coolant being pumped to the coolingmodules at a temperature above the dew point. In traditional coolingsystems, the evaporator is housed in an enclosure that collectscondensation and pumps it to a drain. In addition to cooling, theseunits are often used to dehumidify the space, in which case they areactually controlled to run at temperatures below the room dew point.These units are also usually in the area on the floor or in a mechanicalroom where condensation does not affect sensitive electronic equipment.In some applications for cooling concentrations of electronic equipment,the evaporators are in cooling modules located above the racks thathouse the electronic equipment. The piping to those cooling modules islocated above the electronic equipment. Because of this, it is necessaryto maintain the refrigerant temperature above the room dew pointtemperature so that condensation on the pipes and evaporator isprevented. Such systems are intended for sensible cooling. For at leastthis reason, traditional and VRF systems typically have not beenconsidered for cooling high heat producing spaces because of the issuesinvolved with controlling condensation.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

A variable flow refrigerant system is controlled to modulate the dewpoint temperature of the system in order to prevent condensation fromforming in the room space to be cooled. Suction pressure of the systemis monitored at one or a plurality of suction lines. The suctionpressure corresponds to the lowest refrigerant temperature in the systemand will indicate if the room dew point is being approached. If the loadon one or more of the evaporators increases, the outlet superheat willincrease and the corresponding expansion valve will open to allow moremass flow through the corresponding evaporator. When the expansion valveopens, suction pressure increases, and a variable compressor willrespond by increasing capacity. If the load on one or more of theevaporators decreases, the superheat will decrease as well, causing therespective expansion valve to close. When the expansion valve closes,suction pressure decreases. If the decrease in suction pressure lowersthe corresponding saturation temperature below the room dew point, thecontroller will direct the compressor to unload in order to raise thesuction pressure back to a value with a corresponding saturationtemperature above the dew point.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1. is a multiple evaporator VRF cooling system arranged accordingto various embodiments;

FIG. 2. is a multiple evaporator VRF cooling system arranged accordingto various embodiments;

FIG. 3. is a multiple evaporator VRF cooling system arranged accordingto various embodiments;

FIG. 4. is a multiple evaporator VRF cooling system arranged accordingto various embodiments;

FIG. 5. is a multiple evaporator VRF cooling system arranged accordingto various embodiments; and

FIG. 6 is an example enthalpy diagram detailing operation of an exampleVRF cooling system; and

FIG. 7 is a block diagram indicating for depicting control of the VRFcooling system.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

When an element or layer is referred to as being “on,” “engaged to,”“connected to,” or “coupled to” another element or layer, it may bedirectly on, engaged, connected or coupled to the other element orlayer, or intervening elements or layers may be present. In contrast,when an element is referred to as being “directly on,” “directly engagedto,” “directly connected to,” or “directly coupled to” another elementor layer, there may be no intervening elements or layers present. Otherwords used to describe the relationship between elements should beinterpreted in a like fashion (e.g., “between” versus “directlybetween,” “adjacent” versus “directly adjacent,” etc.). As used herein,the term “and/or” includes any and all combinations of one or more ofthe associated listed items.

Although the terms first, second, third, etc. may be used herein todescribe various elements, components, regions, layers and/or sections,these elements, components, regions, layers and/or sections should notbe limited by these terms. These terms may be only used to distinguishone element, component, region, layer or section from another region,layer or section. Terms such as “first,” “second,” and other numericalterms when used herein do not imply a sequence or order unless clearlyindicated by the context. Thus, a first element, component, region,layer or section discussed below could be termed a second element,component, region, layer or section without departing from the teachingsof the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,”“lower,” “above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

A multiple evaporator VRF system 110 according to various embodiments isdescribed in accordance with FIG. 1. VRF system 110 includes an indoorunit 112 placed in proximity to the room space to be cooled and acondenser unit 114 placed remotely from the room space to be cooled. Invarious embodiments, remote unit 114 is placed outside of the buildinghousing the space to be cooled. Indoor unit 112 in various embodimentsis located in or above a row of computer server racks. A control unit119 includes a compressor and a control circuit. Control unit 119 islocated indoors. In various embodiments, control unit 119 could belocated outdoors. In various embodiments, control unit 119 could belocated in proximity to or remotely from indoor unit 112 or condenserunit 114.

As shown in FIG. 1, indoor unit 112 includes multiple evaporatorcircuits, shown in FIG. 1 as evaporator circuits 116 a, 116 b, . . . ,116 n−1, 116 n. Each evaporator circuit includes a respective expansionvalve 118 a, 118 b, . . . , 118 n−1, 118 n. Expansion valves 118 controlthe application of cooling fluid to respective evaporators 120 a, 120 b,. . . , 120 n−1, 120 n. Expansion valves 118 operate either mechanicallyas thermal expansion valves or electrically powered via a dedicatedcontroller. The expansion valves 118 respond to control evaporatoroutlet superheat. If the load on an evaporator 120 increases, thesuperheat will increase, and the expansion valve 118 will respond byopening to allow more refrigerant flow, which will return the superheatto the desired level. Each evaporator 120 a, 120 b, . . . , 120 nconnects to a respective suction line 122 a, 122 b, . . . , 122 n−1, 122n which connects to a central suction line 124. Evaporators 120,according to various embodiments, are implemented using, by way ofnonlimiting example, conventional fin and tube evaporators ormicrochannel heat exchangers as the evaporating coil. Microchannel heatexchangers offer improved heat removal per unit volume, providing aspace savings. Microchannel heat exchangers require less internal volumewhich facilitates refrigerant charge management because smallerrefrigerant volumes are required. This enables the use of smaller liquidreceivers and suction accumulators.

Central suction line 124 connects to a suction accumulator 126. Suctionaccumulator 126 accumulates liquid refrigerant. To maintain suction oncentral suction line 124, suction is applied to suction accumulator 126through a return or suction input line 128 which connects to an inlet130 of a compressor 132, such as a variable capacity compressor,arranged in control unit 119 of VRF system 110. Variable capacitycompressor 132 receives mechanical drive through input shaft 134.Variable capacity compressor 132 generates suction on suction input line128. Variable capacity compressor 132 also includes an outlet 136 whichconnects to a discharge line 138. Discharge line 138 provides fluid atpressure to a condenser 140.

VRF system 110 also includes a controller 142 that communicates withcompressor 132 via a signal line 144. Similarly, controller 142communicates with each of respective evaporators 120 a, 120 b, . . . ,120 n−1, 120 n via respective signal lines 156 a, 156 b, . . . , 156n−1, 156 n.

Condenser 140 connects to an output or liquid line 150 which is routedto suction accumulator 126 to provide further cooling of fluid flowingthrough liquid line 150. At the output of suction accumulator 126,liquid line 152 connects to each of individual liquid lines 154 a, 154b, . . . , 154 n−1, 154 n. Individual liquid lines are input torespective evaporator valves 118 a, 118 b, . . . , 118 n−1, 118 n.

VRF system 110 is controlled to maintain the refrigerant temperatureabove the room dew point temperature in order to prevent condensationfrom forming in the room space to be cooled. In operation, suctionpressure of the system is monitored at one or any of suction lines 122,central suction lines 124, or return suction input line 128. The suctionpressure corresponds to the lowest refrigerant temperature in the systemand will indicate if the dew point of the system is being approached. Ifthe load on any one or more of evaporators 120 increases, the outletsuperheat will increase, and the respective expansion valves 118 willopen to allow more mass flow through that particular evaporator 120.With the expansion valve opening, suction pressure will increase, andvariable capacity compressor 132 will respond by increasing capacity.

If the load on one or more of the evaporators 120 decreases, thesuperheat will decrease as well, causing the respective expansion valve118 to close. When the respective expansion valve 118 closes, suctionpressure will decrease. If the decrease in suction pressure lowers thecorresponding saturation temperature below the room dew pointtemperature, the controller 142 will direct compressor 132 to unload inorder to raise the suction pressure back to a value with a correspondingsaturation temperature above the dew point. In various embodiments, theactual evaporator pressure can be monitored via signal lines 156 a, 156b, . . . , 156 n−1, 156 n for each of respective evaporators 120 a, 120b, . . . , 120 n−1, 120 n. Controller 142 in various embodiments cancontrol the unloading of compressor 132 via signal line 144. Thus, invarious embodiments, suction pressure is monitored in order to maintaina minimum pressure, and resultant temperature, of the refrigerant in thecircuit.

FIG. 2 is arranged similarly to FIG. 1 with modifications describedherein. Throughout the description, items similarly configured in thefigures will be referred to using similar reference numbers. Forexample, item 112 of FIG. 1 will be referred to as item 212 in FIG. 2.Likewise, for example, condenser 140 of FIG. 1 will be referred to ascondenser 240 in FIG. 2 and condenser 340 in FIG. 3, etc. Items insucceeding figures which are configured similarly to prior figures willnot be described in detail, and the operation of similarly configureditems will not be described in detail. However, differing items oroperations will be described in greater detail as necessary.

FIG. 2 depicts a VRF system 210. VRF system 210 includes a liquidreceiver 260 disposed in liquid line 250 which is shown as an input line250′ to liquid receiver 260 and an output line 250″ at the output ofliquid receiver 260. Output line 250″ connects to the input of suctionaccumulator 226. In FIG. 2, the refrigerant charge is managed with asuction line accumulator 226 and liquid receiver 260. The liquid line250, which is a high pressure and high temperature line, is routedthrough suction accumulator 226 in order to heat and evaporate anyliquid that might be in the accumulator. Liquid receiver 260 provides areservoir that accumulates cooling fluid.

FIG. 3 depicts a VRF system 310 arranged in accordance with anotherembodiment of the present invention. In FIG. 3, the suction accumulator126, 226 of respective FIGS. 1 and 2 has been removed, and centralsuction line 324 forms a continuous return to input 330 of compressor332. In FIG. 3, the refrigerant charge is managed via liquid receiver360. In FIG. 3, the liquid line 350″ output from liquid receiver 360 isin thermal contact with suction line 324 in order to heat the vapor incentral suction line 324 in order to evaporate any liquid that might bein central suction line 324. This reduces the liquid that could reachthe input 330 of compressor 332.

FIG. 4 depicts a VRF system 410 arranged in accordance with anotherembodiment. In FIG. 4, the refrigerant charge is managed with suctionaccumulator 426 and liquid receiver 460. Liquid receiver 460 has anoutput directly to liquid line 452. In this manner, liquid receiver 460does not have an output connected to the input of the heat exchanger ofsuction accumulator 426. The liquid output from liquid receiver 460 doesnot provide a heat exchange function for suction accumulator 426. Invarious embodiments, suction accumulator 426 could include asupplemental heater 462, such as a strap-on heater, which could beoperated when the liquid in return line 424 reaches a predeterminedlevel.

FIG. 5 depicts VRF system 510 arranged in accordance with anotherembodiment. In the embodiment of FIG. 5, refrigerant charge is managedusing only liquid receiver 560. Suction line 524 communicates with input530 of compressor 532 and is applied directly thereto. In the embodimentof FIG. 5, no suction accumulator is shown, which differs from theembodiments of FIGS. 1-2 and 4. In this manner, only liquid receiver 260manages the refrigerant charge.

FIG. 6 depicts one example of an enthalpy diagram for demonstrating anexemplary operation of the various embodiments described herein. As canbe seen in FIG. 6, a saturation curve 670 can be used to demonstrate thestate of the refrigerant. As can be seen, suction pressure is monitoredbecause this pressure corresponds to the lowest refrigerant temperaturein the system as indicated by the saturated liquid line 672. If the loadon an evaporator increases, the expansion valve will open to allow moremass flow through the evaporator. With the expansion valve opening, thesuction pressure will increase and the compressor will respond byincreasing capacity.

FIG. 7 depicts a block diagram 780 for controlling operation of a VRFcooling system in connection with FIG. 1. Control begins at start block782 and proceeds to block 784. Block 784 monitors the suction pressure.Control then proceeds to block 786. At block 786, control proceeds basedon the suction pressure. If the suction pressure is determined to behigh, control proceeds to block 788 which increases the compressoroutput in order to compensate for the high suction pressure. Controlthen proceeds to block 784. If the suction pressure is determined to below at block 786, then control proceeds to block 790 which decreases thecompressor output. Control then proceeds to block 784. Returning toblocks 786, if the suction pressure is unchanged, control returns toblock 784.

Returning to block 790, a decrease in suction pressure can lower thecorresponding saturation temperature below the room dew pointtemperature. In various configurations, having the saturationtemperature below the dew point temperature can generate undesiredcondensation. Accordingly, the suction pressure is monitored andassociated with a corresponding saturation temperature. If the value ofthe suction pressure indicates that the corresponding saturationtemperature is below room dew point temperature, the compressor outputis decreased in order to maintain the suction pressure so that thecorresponding saturation temperature stays above the room dew pointtemperature.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

What is claimed is:
 1. A variable refrigerant flow cooling systemcomprising: a compressor having an inlet and an outlet, the inletgenerating a first pressure, and the outlet generating a second pressurehigher than the first pressure; a condenser having an inlet and anoutlet, the inlet of the condenser communicating with the outlet of thecompressor, the condenser receiving fluid provided by the outlet of thecompressor and removing heat from the fluid; an expansion valve havingan inlet and an outlet, the inlet of the expansion valve communicatingwith the outlet of the condenser, the expansion valve enabling expansionof the liquid at its outlet and varying a flow of liquid between itsinlet and outlet; an evaporator having an inlet and an outlet, the inletof the evaporator communicating with the outlet of the expansion valve,the evaporator absorbing heat into the fluid as the fluid passes fromits inlet to outlet; and a controller monitoring a pressure at theoutlet of the evaporator and varying the output of the compressor inaccordance with the monitored pressure, wherein the monitored pressureindicates a saturation temperature and the control maintains thesaturation temperature above a dew point temperature.
 2. The variablerefrigerant flow system of claim 1 wherein the expansion valve isresponsive to the superheat of the evaporator.
 3. The variablerefrigerant flow system of claim 1 further comprising a suction linecommunicating with the outlet of the evaporator at a first end and theinlet of the compressor at a second end for fluid flow between theoutlet of the evaporator and the inlet of the compressor.
 4. Thevariable refrigerant flow system of claim 3 further comprising anaccumulator interposed in the suction line between the evaporator andthe compressor.
 5. The variable refrigerant flow system of claim 4wherein fluid flowing from the outlet of the condenser to the inlet ofthe expansion valve passes through the accumulator in order to reducethe temperature of the liquid flowing therethrough.
 6. The variablerefrigerant flow system of claim 1 further comprising a liquid receiverhaving an inlet and an outlet, the inlet of the liquid receivercommunicating with the outlet of the condenser and the outlet of theliquid receiver communicating with the expansion valve.
 7. The variablerefrigerant flow system of claim 6 wherein the outlet of the liquidreceiver is in thermal communication with a suction line interconnectingthe output of the evaporator with the input of the compressor, therebycooling the liquid output by the liquid receiver prior to input to theexpansion valve.
 8. The variable flow refrigerant system of claim 1further comprising: a plurality of expansion valves, each having aninlet and an outlet, the inlet of the each expansion valve communicatingwith the outlet of the condenser, each expansion valve enablingexpansion of the liquid at its outlet and varying a flow of liquidbetween its inlet and outlet; and a plurality of evaporators, eachevaporator having an inlet and an outlet, the inlet of each evaporatorcommunicating with a respective outlet of the expansion valve, eachevaporator absorbing heat into the fluid as the fluid passes from itsinlet to outlet.
 9. A variable refrigerant flow cooling systemcomprising: a compressor generating a first pressure and a secondpressure higher than the first pressure; a condenser communicating withthe second pressure, the condenser receiving fluid provided at thesecond pressure and reducing the temperature of the fluid; a liquid linecommunicating with the condenser and receiving the liquid; an expansionvalve communicating with the liquid line, the expansion valve enablingexpansion of the fluid in the liquid line; an evaporator communicatingwith the fluid output by the expansion valve, the fluid absorbing heatas it passes through the evaporator; a vapor line communicating with theevaporator and returning fluid output from the evaporator to thecompressor; and a controller monitoring a pressure in the vapor line andvarying the output of the compressor in accordance with the monitoredpressure, wherein the monitored pressure indicates a saturationtemperature of the vapor line, and the output of the compressor isvaried to maintain the saturation temperature above a dew pointtemperature for the vapor line.
 10. The variable refrigerant flow systemof claim 9 wherein the expansion valve is responsive to the superheat ofthe evaporator.
 11. The variable refrigerant flow system of claim 9further comprising an accumulator interposed in the vapor line betweenthe evaporator and the compressor.
 12. The variable refrigerant flowsystem of claim 11 wherein liquid line passes through the accumulator inorder to reduce the temperature of the fluid flowing therethrough. 13.The variable refrigerant flow system of claim 9 further comprising aliquid receiver interposed in the liquid line.
 14. The variablerefrigerant flow system of claim 13 wherein the output of the liquidreceiver is in thermal communication with the vapor line, therebycooling the liquid output by the liquid receiver prior to input to theexpansion valve.
 15. The variable flow refrigerant system of claim 9further comprising: a plurality of expansion valves communicating withthe liquid line, each expansion valve enabling expansion of the fluid inthe liquid line; and a plurality of evaporators communicating with thefluid output by a respective expansion valve, the fluid absorbing heatas it passes through the evaporator.
 16. A method for controlling anoutput of a compressor in a cooling system comprising: providing anevaporator; monitoring a suction pressure at the output of theevaporator; comparing the suction pressure against a predeterminedthreshold, wherein the predetermined threshold is determined inaccordance with a saturation temperature of a fluid output by theevaporator; and decreasing the output of the compressor if the suctionpressure is below the predetermined threshold.
 17. The method of claim16 further comprising increasing the output of the compressor if thesuction pressure is above a predetermined threshold.
 18. The method ofclaim 17 further comprising maintaining the suction pressure unchangedif the suction pressure is equal to the predetermined threshold.
 19. Themethod of claim 16 further comprising maintaining the suction pressureunchanged if the suction pressure is equal to the predeterminedthreshold.